1. Field of the Invention
The present invention relates to an optical scanning device, and more particularly to an optical scanning device that emits a plurality of light beams concurrently to a photosensitive member.
2. Description of Related Art
A multi-beam scanning device disclosed by Japanese Patent Laid-Open Publication No. 2007-168299 (Reference 1) is an example of optical scanning devices having measures to reduce density unevenness due to reciprocity failure. FIG. 10 shows arrangement of light emitting elements in a light source section 500 of the multi-beam scanning device disclosed by Reference 1.
As shown by FIG. 10, in the light source section 500 of the multi-beam scanning device disclosed by Reference 1, a multiple of light emitting elements 502 are arranged in a matrix of four rows and eight columns. The longer sides of the matrix slant slightly from a main-scanning direction so that beams emitted from the light emitting elements 502 will be scanned on different scanning lines. Also, among the eight light emitting elements 502 in the first row, four light emitting elements 502 located in the columns of even numbers, actually, do not emit light. Likewise, among the eight light emitting elements 502 in the fourth row, four light emitting elements 502 located in the columns of odd numbers do not emit light. With the light source section 500 of this structure, the multi-beam scanning device disclosed by Reference 1 can irradiate a photosensitive member with a plurality of beams concurrently while reducing density unevenness due to reciprocity failure.
An ordinary optical scanning device for scanning a photosensitive member with a plurality of beams concurrently is structured such that an area irradiated with beams at the time of the Lth scanning motion and an area irradiated with beams at the time of the (L+1)th scanning motion will not overlap. However, when such an ordinary optical scanning device is used, actually, a scanning line located at the most downstream in the sub-scanning direction during the Lth scanning motion is inevitably superimposed during the (L+1)th scanning motion. Then, due to reciprocity failure, this scanning line becomes thicker than the other scanning lines, and density unevenness is caused. Thus, when one scanning line is especially thicker, the line is apt to be recognized as a thick stripe by users.
The multi-beam scanning device disclosed by Reference 1 is so structured that the area irradiated with beams during Lth scanning motion and the area irradiated with beams during (L+1)th scanning motion overlap with each other. More specifically, scanning lines on which the beams emitted from the four light emitting elements 502 in the first row at the time of the (L+1)th scanning motion are located among scanning lines on which the beams emitted from the four light emitting elements 502 in the fourth row at the time of the Lth scanning motion.
Therefore, since the four scanning lines on which the beams emitted from the four light emitting elements 502 in the fourth row are scanned during the Lth scanning motion are superimposed, these scanning lines become thicker due to reciprocity failure. On the contrary, the four scanning lines on which the beams emitted from the four light emitting elements 502 in the first row are scanned during the (L+1)th scanning motion are thinner than the four scanning lines on which the beams emitted from the four light emitting elements 502 in the fourth row are scanned during the Lth scanning motion. Further, the thicker scanning lines, which are made by the beams emitted from the four light emitting elements 502 in the fourth row during the Lth scanning motion, and the thinner scanning lines, which are made by the beams emitted from the four light emitting elements 502 in the first row during the (L+1)th scanning motion, are arranged alternately. Thereby, the density unevenness in an image formed by use of the multi-beam scanning device disclosed by Reference 1 is seen gentle, compared with an image formed by an ordinary optical scanning device. Thus, in an image formed by use of the multi-beam scanning device disclosed by Reference 1, density unevenness due to reciprocity failure is less likely to be recognized as stripes, compared with an image formed by an ordinary optical scanning device.
However, in the multi-beam scanning device disclosed by Reference 1, some of the light emitting elements 502 in the first row and in the fourth row actually do not emit light. Therefore, although the light source section 500 consumes a space for 32 light emitting elements, actually only 24 light emitting elements 502 emit light. Thus, the multi-beam scanning device disclosed by Reference 1 has a problem that the light source section 500 consumes a space unnecessarily and becomes large. When the light source section 500 is large, the beams emitted from the light source section 500 will pass through the peripheral portions of a collimator lens and a cylindrical lens as well as the central portions thereof. The peripheral portions of the collimator lens and the cylindrical lens have greater aberrations than the central portions thereof. Therefore, it is desired that the light source section 500 is downsized such that the beams will pass through the central portions of the collimator lens and the cylindrical lens.